Power electronics and electric power systems for many practical applications often require storage devices (for example, capacitors) with high energy densities to enable miniaturization of circuits and devices. For example, in the design of implantable cardiac defibrillators, energy, volume and mass are critical features. One component important to the optimization of these features is the high voltage capacitor used to store the energy required for defibrillation. Such a capacitor tends to be bulky as it is required to deliver energy in the range of about 25 to 40 Joules. It is preferable if the volume and mass of this capacitor can be reduced as this can be beneficial to patient comfort. It can also minimize the occurrence of complications due to erosion of tissue around the storage device. Reductions in the size of the capacitor can also allow for a balanced addition of volume to the battery, thereby adding functionality to the circuit or device.
For many other conventional power converters and pulse power systems, the capacitor size may be a concern since capacitors often occupy more than 50% of the overall volume. Thus, it is desirable to reduce the volume and mass of capacitors in power electronics and electric power systems without reducing the amount of deliverable energy.
Ceramic-based dielectric materials generally have a relatively low breakdown field. As such there may be a high chance of catastrophic failure under a high electric field when ceramic-based dielectric materials are used. Thus, these ceramic-based dielectric materials have a low electric energy density (approximately 1 J/cm3). Conventional polymeric dielectric materials, such as polypropylene (PP), polyethylene terephthalate (PET), and polycarbonate (PC) have a high breakdown electric field and have been used widely for high energy density capacitors. However, these polymers are linear dielectrics with very low dielectric constants, thus limiting the capacitance values of the capacitors. A realistic energy density of these polymers is typically not significantly higher than 1 J/cm3.
There have been a number of recent attempts to use poly(vinylidene fluoride) (PVDF) polymers and copolymers or their modified counterparts for high energy storage capacitor applications.
Even though attempts to use poly(vinylidene fluoride) (PVDF) polymers and copolymers or their modified counterparts have led to improved discharged energy densities with highest values up to 10-25 J/cm3, such levels may still not be sufficient and further improvement in the discharged energy densities is desired.
Vinylidene fluoride (VDF) oligomer typically comprises less than a few hundred units of —CH2CF2—, which has much smaller molecular weight than poly(vinylidene fluoride) (PVDF) polymers whereas VDF co-oligomer typically comprises less than a few hundred units of VDF and other monomers such as trifluoroethylene (TrFE). VDF oligomer or co-oligomer films may be useful for high energy storage capacitor applications.
It has been reported that a VDF oligomer film can be produced by thermal evaporation process. However, β-phase VDF oligomer films were only achieved using very specific substrates or at low temperature. The obtained VDF oligomer films do not possess the performance as desired for high energy density storage because the polarization hysteresis loops indicate small linear polarization and the largest polarization Pm is rather low. There have been several proposals for the preparation of VDF oligomer films from solutions using the technique of Langmuir-Blodgett deposition or solution casting. However, the solution-derived VDF oligomer films have poor morphologies or structures, and do not show any useful electrical properties.
There is hence a need for a dielectric material with improved electrical properties, particularly improved energy density, and a method of forming such a dielectric material.